For a photoelectrode of a Dye-Sensitized Solar Cell (DSSC), a porous film onto which a photosensitive dye such as a ruthenium metal complex has been adsorbed and which is formed of an oxide semiconductor such as titanium oxide is used (PTL 1). In the conventional method for forming the porous film, slurry or paste containing particles of an oxide semiconductor is coated onto a base material, and the resultant is baked at a temperature less than or equal to a melting point of the oxide semiconductor, whereby a film in which the particles of the oxide semiconductor come into contact with one another with a weak force is formed. Generally, before coating, a binder such as ethyl cellulose is mixed in the slurry or paste so as to adjust the viscosity of the slurry or paste, improve porosity of a film to be formed, and form a porous film having an increased adsorption area of the photosensitive dye.
There is a known method that treats the surface of fine particles constituting the porous film to improve a photoelectric conversion efficiency (also called a generating efficiency) of a dye-sensitized solar cell. As a typical example thereof, a method, in which the porous film formed of titanium oxide that is formed as described above is dipped into an aqueous titanium tetrachloride solution, pulled up from the solution, and subjected to heating treatment to form a new titanium oxide film on the porous film, is known (NPL 1). The titanium oxide film is considered to play a role of forming a clean surface of the porous film and enhancing the adhesive force between the titanium oxide particles constituting the porous film.